Mark A. Smith of the University of Arizona is supported by the Experimental Physical Chemistry Program to continue his studies on the mechanism and dynamics of gas phase chemical reactions at temperatures between 1 and 300K. The primary goal is to further the understanding of very low energy collision phenomena and to obtain absolute reaction rates and product branching ratios needed for the modeling of planetary atmospheric, cometary, circumstellar and interstellar chemical environments. He will investigate radical-molecule and molecule-molecule collision processes near 1K using a free jet-based flow reactor, and measure rates, branching ratios and product identities over a wide range of temperatures and pressures using laser-induced fluorescence-based and mass spectrometer-based Laval flow reactors. The wide temperature range of these studies encompasses those prevalent in interstellar clouds, and many of the reactions to be investigated are particularly important in that environment. For example, there is an enhancement of deuterium atoms in molecules in these regions, which may be due to grain mechanisms, fractionation in ion-molecule reaction pathways or neutral fractionation. Reactions must be studied to determine which alternative is viable. Our general knowledge of intermolecular interactions at low energies is poor because experimental methods have not previously existed to permit studies at temperatures below about 250K. These studies will break new ground in physical chemistry at low temperatures.
